The invention relates to an inductive fault current limiter, comprising                a normally conducting primary coil assembly with a multiplicity of turns, and        a superconducting, short-circuited secondary coil assembly,wherein the primary coil assembly and the secondary coil assembly are disposed at least substantially coaxially with respect to each other and at least partially interleaved in each other.        
Such an inductive fault current limiter is known from EP 2 402 961 A1.
Fault current limiters are used to limit the current drawn from a source in case of short circuits. With superconducting fault current limiters, comparatively high currents, for example, in public electrical supply networks, can be made safe.
Superconducting fault current limiters of the direct current type comprise, for example, a superconducting conductor section, through which the current to be limited flows. If the current rises due to a short circuit, the critical current in the superconducting conductor section is exceeded so that it becomes normally conducting (“quench”), which greatly increases the resistance. This causes the current flowing through the conductor section to fall correspondingly. This type of fault current limiter is termed the resistive type.
Superconducting fault current limiters for alternating current applications can be implemented both as resistive and as inductive fault current limiters. Inductive fault current limiters are essentially constituted by inductively coupled turns. The alternating current to be limited flows through a normally conducting primary coil assembly (“primary side”) and, in a superconducting secondary coil assembly (“secondary side”), a current is induced that counteracts the magnetic flux produced by the current in the primary coil assembly. This largely cancels out the inductance of the primary coil assembly. If the alternating current to be limited in the primary coil assembly rises due to a short circuit, the induced current in the secondary coil assembly also rises until the critical current is exceeded in the secondary coil assembly and the secondary coil assembly becomes normally conducting (“quench”). In the normally conducting condition of the secondary coil assembly, its current only compensates for a small part of the magnetic flux of the primary coil assembly, which, in turn, raises the inductance of the primary coil assembly. This increase in inductive resistance limits the current through the primary coil assembly.
An inductive fault current limiter is known from EP 2 402 961 A1, in which a secondary coil, which comprises a plurality of turns radially disposed within the primary coil, and which is constituted as a superconducting, short-circuited ring, is disclosed. From DE 10 2010 007 087 A1, an inductive fault current limiter is known, in which a cryostat with a secondary coil is disposed inside a primary coil. From DE 196 28 358 C1 (and from U.S. Pat. No. 5,140,290 A, U.S. Pat. No. 6,795,282 B2, U.S. Pat. No. 7,586,717 B2, U.S. Pat. No. 5,694,279 A, DE 195 24 579 A1, U.S. Pat. No. 5,379,020 A), it is known that a primary coil and a secondary coil may be disposed side by side and coupled via a ring-shaped iron core.
The efficiency of an inductive fault current limiter is measured according to the inductance ratio of the primary coil assembly in a superconducting secondary coil assembly (“normal operation”) and in a normally conducting secondary coil assembly (“fault operation”). During fault operation, the current in the primary coil assembly should be very much reduced as compared with normal operation and in correspondence with a high inductance ratio.
The object of the invention is to provide a fault current limiter with an increased inductance ratio.